The invention concerns a volumetric compressor provided with a delivery adjustment device and in particular a screw compressor comprising a casing in which it is possible to identify a suction chamber equipped with a suction valve and a delivery chamber equipped with a delivery valve, between which a pair of screw rotors meshing with each other is included. At the bottom of the casing there is a pan for the lubrication oil.
It is known that the volumetric screw compressors described above are equipped with a delivery adjustment unit comprising a slide valve that cooperates externally with the rotors and is set in motion by a fluid-operated actuator according to a longitudinal direction parallel to the longitudinal axis of the rotors themselves.
The fluid-operated actuator is provided with an active chamber fed with the oil coming from the pan in order to obtain the sliding movement of a piston positioned in the active chamber and provided with a rod that connects it to the slide valve.
On the liner and on the bottom of the actuator there is a plurality of flow paths connected to the same number of drain pipes conveying the oil from the active chamber of the actuator to the suction chamber of the compressor.
In particular, each drain pipe is equipped with an on-off valve and the paths for communication with the active chamber, to which the pipes are connected, are arranged as follows: one on the bottom and the others, positioned on the liner, aligned parallel to the piston sliding direction and having different axial distances with respect to the bottom.
In this way, by selectively opening and closing the valves, it is possible to maintain different quantities of oil in the active chamber of the actuator, in such a way as to arrange the piston, and therefore the slide valve connected to it, in different axial positions with respect to the rotors.
In this way, the compressor's suction is controlled and the delivery of the same is modified.
According to the above, it is obvious that the degree of control of the compressor's delivery depends on the position of the flow paths of the actuator and on what on-off valves are opened and what remain closed.
A volumetric screw compressor of the type mentioned above is described in the European Patent application EP 1 072 796 in the name of Bitzer Kühlmaschinenbau GmbH, according to which an electric/electronic control device, connected to the actuators of the on-off valves of the drain pipes, controls the opening and closing of the valves themselves in such a way as to control the delivery of the compressor, according to the user's needs.
The above mentioned control device manages the opening and closing of the above mentioned valves according to different modes, in such a way as to control the compressor's delivery in steps or continuously.
The volumetric compressor described in the patent application mentioned above has some recognized drawbacks.
A first recognized drawback is constituted by the fact that the on-off valves are controlled electrically and, to adjust the delivery, a suitable control device acts on the solenoids that control more than one valve.
Therefore, in case of failure of the control device, the operation of the adjustment unit is stopped completely.
Another recognized drawback is constituted by the long time required for the repairs in case of failure of the control device.
The present invention aims to overcome the drawbacks listed above.